selftests: breakpoints: return Kselftest Skip code for skipped tests

When step_after_suspend_test is skipped because of unmet dependencies
and/or unsupported configuration, it exits with error which is treated
as a fail by the Kselftest framework. This leads to false negative result
even when the test could not be run.

Change it to return kselftest skip code when a test gets skipped to clearly
report that the test could not be run.

Change it to use ksft_exit_skip() when a non-root user runs the test and
add an explicit check for root and a clear message, instead of failing
the test when /sys/power/state file open fails.

Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>